I've heard of the Nani SS absorbing water and causing problems. I only spritz mine, and get good results. Swarf build up can be a problem when lapping. Do so under running water. The first thing that came to my mind was your using too much pressure; However, If you can, I would see if the vendor will give you another stone. If you don't do that. It'll always be in the back of your mind..."why didn't I do that?

Have you tried building a slurry on the 12k before you use it. Take a DMT C if you have one, and do a couple of figure 8's, Try honing, 10 lapps, dilute, 5, dilute...5, test...This is part of Glen's system...I've gotten some really sharp edges this way...